8.1.6Space Physics
Circular Orbits
5 min read 5 parts Lesson 6 of 10
Section 1 of 5Forces in Circular Orbits
A circular orbit needs a resultant force that always points towards the centre of the circle.
Centripetal force
- Centripetal force A circular orbit needs a force acting towards the centre, which for a satellite or planet is provided by gravity.
Constant speed
- Constant speed An object in a circular orbit moves at a constant speed, but its velocity changes because its direction changes.
Direction
- Direction The gravitational force is always directed towards the centre of the orbit, at right angles to the velocity.
Orbit radius
- Orbit radius A larger orbit radius means a longer orbital period, because the object must travel a greater distance in each orbit.
Section 2 of 5Orbit Types
Different orbits suit different satellites, depending on their height and period.
1
A low Earth orbit is close to the Earth and has a period of about 90 minutes, as for the International Space Station.
2
A geostationary orbit lies above the equator and has a period of 24 hours, so the satellite stays above one point on the Earth.
3
The Moon orbits the Earth with a period of about 27 days, which is much longer than the period of a low orbit.
| Orbit | Typical use | Period |
|---|---|---|
| Low Earth orbit | Earth observation and the International Space Station | About 90 minutes |
| Geostationary orbit | Communications and weather satellites | 24 hours |
| Moon | Natural satellite of the Earth | About 27 days |
Section 3 of 5Speed, Radius and Period
The orbital speed is linked to the radius and period by the circumference of the orbit.
Speed, Radius and Period
Circumference
- Circumference In one orbit the object travels a distance equal to the circumference, 2πr.
Orbital speed
- Orbital speed Dividing the circumference by the period T gives the orbital speed, v = 2πr ÷ T.
Central body
- Central body The orbital speed depends on the mass of the body being orbited and on the orbit radius.
Closer orbits
- Closer orbits Satellites nearer the planet must travel faster to remain in orbit, and they have shorter periods.
Section 4 of 5Worked Example
Question
- A satellite orbits the Earth at a radius of 7.0 × 10⁶ m with a period of 5400 s.
- Calculate its orbital speed.
- Distance travelled in one orbit = 2πr = 2π × 7.0 × 10⁶ m = 4.40 × 10⁷ m (3 significant figures).
- Orbital speed = distance ÷ period = 4.40 × 10⁷ ÷ 5400 s.
- Orbital speed = 8.1 × 10³ m/s (2 significant figures).
Answer The orbital speed of the satellite is about 8.1 × 10³ m/s.
Section 5 of 5Exam Checklist & Tips
8.1.6 – Key Points for the Exam
- Orbit The curved path followed by a satellite or planet around a larger body.
- Centripetal force A resultant force acting towards the centre of a circular path.
- Period The time taken for one complete orbit or cycle.
- Geostationary orbit A circular orbit above the equator with a period of 24 hours.
- Orbital speed The speed at which a satellite travels around its orbit.
Common Mistakes
- Stating that satellites in a circular orbit have no forces acting on them;
- gravity acts as the centripetal force.
- Saying that orbital speed changes continuously in a circular orbit;
- the speed is constant but the velocity changes.
- Using the radius instead of the circumference in the speed calculation, which leaves out the factor of 2π.